1. Field of the Invention
This invention relates to an evaporative fuel-purging control system for an internal combustion engine having an evaporative emission control system.
2. Prior Art
Conventionally, evaporative emission control systems have been widely used in internal combustion engines for automotive vehicles, which operate to prevent evaporative fuel (fuel vapor) from being emitted from a fuel tank into the atmosphere, by temporarily storing evaporative fuel from the fuel tank in a canister, and purging same into the intake system of the engine. Purging of evaporative fuel into the intake system causes instantaneous enriching of an air-fuel mixture supplied to the engine. If the purged evaporative fuel amount is small, the air-fuel ratio of the mixture will then be promptly returned to a desired value, with almost no fluctuation.
However, if the purged evaporative fuel amount is large, the air-fuel ratio of the mixture fluctuates. In order to prevent fluctuations in the air-fuel ratio due to purging of evaporative fuel (fuel vapor), there have been proposed (i) an air-fuel ratio control system which is adapted to increase the control gain of air-fuel ratio control during purging of evaporative fuel from the canister into the intake pipe or within a predetermined time period after the start of the purging (Japanese Provisional Patent Publications (Kokai) Nos. 62-139941 and 63-71536, and Japanese Provisional Utility Modeal Publication No. 63-190541), and (ii) an air-fuel ratio control system which is adapted to progressively increase the valve opening of a purge control valve which controls the flow rate of a mixture supplied from the canister to the intake system of the engine, when the air-fuel ratio of the mixture detected by an air-fuel ratio sensor arranged in the exhaust system of the engine shows a value on a lean side with respect to a desired or stoichiometric air-fuel ratio (Japanese Provisional Patent Publication (Kokai) No. 2-245461).
Generally, when the concentration of evaporative fuel in a mixture purged into the intake system of the engine changes, the rate of change in the air-fuel ratio of the mixture relative to change in the valve opening of the purge control valve changes accordingly. That is, insofar as the amount of change in the valve opening of the purge control valve remains constant, the higher the evaporative fuel concentration, the greater the amount of change in the air-fuel ratio. Therefore, if the speed at which the purge control valve opens and closes, i.e. the change rate of the valve opening is set at a constant value irrespective of the evaporative fuel concentration, it will result in temporary overriching of the air-fuel ratio when the evaporative fuel concentration is high, whereas when the evaporative fuel concentration is low, the amount of fluctuation of the air-fuel ratio is so small that the air-fuel ratio control for suppression of fluctuations of the air-fuel ratio due to purging cannot be effected with sufficient responsiveness by changing the valve opening of the purge control valve.
The system (i) does not contemplate the closing and opening speed or change rate of valve opening of the purge control valve and therefore suffers from the above-mentioned disadvantage.
According to the system (ii), when the detected air-fuel ratio is on a rich side wi&h respect to the desired or stoichiometric air-fuel ratio, the valve opening of the purge control valve is held as it is. In &his sense, the opening speed of the purge control valve is changed depending upon the detected air-fuel ratio. However, the system (ii) does not directly detect the purging amount of evaporative fuel for controlling the valve opening speed and therefore cannot fully suppress fluctuations in the air-fuel ratio due to purging.